Aromatic polyesters such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) are known to exhibit well-balanced and desirable physical properties, such as thermal resistance, mechanical strength and gas impermeability. As a result, aromatic polyesters are widely used as engineering plastics to form a number of articles used in various end-use applications.
Recently, composite sheets comprised of resin films laminated one to another and/or to a backing sheet (e.g. paper) have been proposed so as to attain beneficial properties--for example, to take advantage of the excellent film-forming and gas barrier properties of aromatic polyesters generally.
One problem associated with aromatic polyesters however is that, since they are crystalline resins, the tear strength is reduced significantly by thermal history conditions, such as heat treatments after film formation and/or exposure to a heated environment during use. Therefore, aromatic polyester films cannot be subjected to any after-treatments which would expose them to heat (e.g. heated drying) and/or be used in a high-temperature environment. As a result of this problem, the field of use for films formed of aromatic polyesters has been limited.
The present invention is therefore directed to polyester films which significantly minimize (if not eliminate) the problems associated with the use of conventional polyester films. That is, the present invention is directed to polyester resin films comprised of a polyalkylene terephthalate polyester and a bisoxazoline compound blended therewith in amount effective to improve the tear-strength retention after thermal treatment of the resulting film. Thus, it has been found that the incorporation of a bisoxazoline compound with a polyalkylene terephthalate polyester imparts excellent tear-strength retention even after the film has been subjected to thermal history conditions.
In preferred forms, the present invention relates to a polyester film having a thickness of between 10 to 500 .mu.m and comprised of 100 parts by weight of a polyester consisting of alkylene terephthalate groups as main recurring units in the polymer chain, and between 0.1 to 5 parts by weight of a bisoxazoline compound. The polyester films according to the present invention as noted above exhibit excellent tear-strength tension even when subjected to thermal history conditions and thus may be used by themselves in various end-use applications (e.g. food packaging). However, the films of the present invention may also be laminated to another different resin film and/or paper to form a composite sheet having specific properties attributable to each layer forming the composite structure.
Further aspects and advantages of this invention will become more clear to the reader after careful consideration is given to the detailed description of the preferred exemplary embodiments thereof which follow.